In rotary-draw tube-bending the tools make the bend. Indeed, that is why the process is often called “mandrel bending.” Despite the many advances in tube-bending machinery, the rotary-draw process is mechanically identical to what it was a half-century ago when modern tube-bending tools made their first appearance. Therefore, the central importance of the tools in making repeatable high-quality tube bends back then remains just as important today. Small variations or errors in the design, manufacture, or set-up of rotary-draw tools can lead to loss of process control, poor bend quality, abbreviated tool life, and other headaches.
FIG. 1 illustrates a cross-sectional view, through the plane of bend, of the operation of a wiper die assembly 100 that is known in the prior art. The wiper die 101 is a block machined to fit the gap between a bend die 103 and the back tangent 105 of the tube 107 to be bent. The essential element of the wiper die 101 is the knife-like feathered edge 109, which is formed by the intersection of two curved surfaces of the wiper die 101: [1] The bore into which the back tangent 105 of the tube 107 nests, and [2] the radius face against which the bend die 103 bears. Typically, the wiper die 101 is a two-piece assembly comprised of a wiper holder 111 and a wiper insert 113, which features the feathered edge 109. The wiper insert 113 is disposable. Alternatively the wiper die 101 can be a solid block.
The bore of a wiper die will vary with the shape and size of the tube to be bent. The radius face will vary for the same reasons plus the radius of the tube bend. The material of the wiper die is usually metal, although plastic and wood have been used. Occasionally the bore, and less often the radius face, is plated or coated to reduce friction during bending. The material of the wiper die is determined by the material of the tube with durability and friction the primary considerations. Other than size and material, there are no rigid specifications for a wiper die. Most of the tool is nothing but mass to support the feathered edge, to provide sufficient surface area for mounting the wiper die to the tube-bending machine, and in the case of high-pressure tube-bending to act as a backstop for the pressure the tube-bending machine applies to the tube during the rotary-draw tube-bending process. Although the feathered edge is the essential element of a wiper die, no design standard has emerged for it.
The wiper die 101 serves two functions in the rotary-draw tube-bending process. The first is to prevent a hump from forming at the inward half of the back tangent 105 immediately behind the line of tangency 117 when the tube 107 comes to rest at the end of the bending process. As the tube 107 is being drawn around the bend die 103 to form the bend 115, it becomes plasticized at the point of bend 119, which is the region of the tube 107 both ahead of and behind the line tangency 117 being formed into the bend 115. The plasticized material behind the line of tangency 117 continuously flows into the curve of the bend die cavity 103 that is sweeping away from the back tangent 105 of the tube 107, thus forming a hump. As this humped material is draw through the line of tangency 117 it is flattened out. However, if this deformation exceeds the elasticity of the tubing material, the hump or a series of small humps will set in the back tangent 105. Fixturing a wiper die 101 in the gap between the bend die 103 and the back tangent 105 of the tube 107 stops the deformation from reaching that point by blocking the flow of the material. Because all tubing materials have some elasticity, i.e., the property of resuming its original shape when stress is relieved, it is not necessary to fixture a wiper die 101 so that it fills the entire gap to prevent the formation of this hump. As seen in FIG. 1, a wiper can be “raked” as shown by angling the back end of the wiper die 101 from point Ra (i.e., “zero-rake”) and point Rb so that it blocks only that amount of the flow that would exceed the elasticity of the material. The advantage of this technique is longer tool life. In instances where a worn wiper die was set at little or zero-rake, the bore immediately behind the feathered edge 109 is “dished” out from blocking the entire flow of material. This dishing reduces the usable life of a wiper die.
However, raking a wiper die to extend its life can be at odds with its second function, which is full containment of the tubing material at the point of bend when bending under high pressure. Normally, high pressure as radially applied by the pressure die (not shown) against the back tangent 105 of the tube 107 opposite of the wiper die 101 is not necessary in most rotary-draw tube-bending applications. However, higher pressures often cannot be avoided for bending materials such as stainless steel or titanium or even mild steel on an extremely tight bend radius 115. These materials resist the compression that occurs as the intrados of the tube bend 115 (approximately the wall of the inward half of the tube 107) thickens during the bending process. If the flow of material is not completely contained by tooling at the point of bend 117—i.e., the mandrel (not shown) inside in the tube 107, the pressure die (not shown) over the outward half of the back tangent 105, the bend die over the inward half of the tube bend 115, and the wiper over the inward half of the back tangent 105—the compression will buckle the intrados.
FIGS. 2A and 2B illustrate a side view and end view of a traditional solid-body wiper die 200 as used in the prior art. The solid-body wiper die is a solid block 201 with one or more tapped holes 203 at its back end for mounting it to a post or other fixturing device of the tube-bending machine (not shown). The front end is a compound curved face formed by the arc of the bore 205, into which the back tangent of the tube nest, swept along the arc of the radius face 207, which mates with the bend die cavity, starting from the feathered edge 209 to the top of the wiper block 201. Typically the tapped holes 203 do not extend into the bore 205, thus there are no interruptions to the smooth surface of the bore. Similarly, the radius face 207 is an uninterrupted smooth surface. Therefore, therefore are no discontinuities in these two surfaces which act as bearings—to wit, the bore 205 against the radial pressure of the pressure die as applied through the back tangent of the tube and the radius face 207 against the bend die cavity—when a solid-body wiper die 200 is mounted at zero-rake for high-pressure tube-bending. For this reason, along with its generally large mass, the solid-body wiper die 200 is preferred for high-pressure tube-bending.
FIGS. 3A and 3B illustrate a side view and end view of a standard inserted wiper die 300 as used in the prior art. FIG. 3C is a side view of the wiper insert component 303 of that assembly 300. In contrast to the solid-body wiper die 200, the inserted wiper die 300 is a two-piece unit consisting of a wiper holder 301 and a wiper insert 303. The insert 303 is attached to the holder by means of a screw 305. The wiper holder 301 functions in a manner similar to the block 201 of the solid-body wiper die 200. The back end of it has one or more tapped holes 307 to mount the entire wiper assembly 300 to the tube-bending machine. It otherwise does no work during the rotary-draw tube-bending process. The only working surfaces of the inserted wiper die are those of the bore 309 and the radius face 311 of the wiper insert 303. Together these surfaces form the feathered edge 313 of the wiper insert 303. When one of these features wears out, the wiper insert 303 is detached from the wiper holder and replaced with a new one. Replacing an insert is less costly than either re-machining the worn bore 205, radius face 207, or feathered edge 209 of a solid-body wiper 200 or disposing of it. For this reason, the inserted wiper die 300 is preferred for all rotary-draw tube-bending except high-pressure tube-bending.
The inserted wiper die 300 lacks the mass and uninterrupted working surfaces of the solid-body wiper die 200. Because the only function of the wiper holder 301 is to act as a fixture for the wiper insert 303, its bore 315 and radius face 317 are offset from the corresponding surfaces 309, 311 of the insert 303 so as to not incur unnecessary wear. Also because of this limited function the holder 301 has only the mass necessary to support the insert 303. A consequence of this reduced bulk of the holder 301 relative to the block 201 of a solid-body wiper die 200 is that its tapped holes 307 go through its body into the bore 315, which are additional interruptions to the overall continuity of the bore 309, 315 of the inserted wiper die 300. Because of the lack of mass and interrupted working surfaces of the inserted wiper die 300, it is not suited for high-pressure tube-bending.
Therefore, the need exists for a wiper die that incorporates both the performance of the solid-body wiper 200 and the economy of the inserted wiper die 300. The present invention overcomes the shortcomings of all other wiper dies and methods of use as an inserted wiper die with continuous working surfaces and sufficient mass necessary for effective performance in high-pressure tube-bending.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.